The present invention relates to a transmission assembly with a mechanical brake to prevent back driving and in particular to a transmission assembly for use in a power driven vehicle seat assembly wherein the brake prevents unintended movement of the seat due to back driving of the transmission assembly caused by application of forces to the seat assembly.
Vehicle seat assemblies are typically provided with a horizontal adjuster to enable fore and aft adjustment of the seat position relative to the vehicle body. Many seat assemblies have a motor driven power adjustment mechanism which moves the seat fore or aft upon actuation of a control switch. A power seat adjuster typically includes an elongated lead screw, or threaded shaft, rotatably attached to the moving part of the seat assembly. A nut, having a threaded bore, is attached to the lower, fixed portion, of the seat assembly with the lead screw threaded therethrough. An electric motor has an output shaft that is coupled to the lead screw through a transmission assembly. Upon actuation of the motor, the lead screw is rotated, causing the lead screw to move axially through the nut, changing the position of the movable portion of the seat assembly.
In a power adjustment mechanism, it is desirable to minimize noise and vibration produced during seat adjustment. One way to reduce noise and vibration is to reduce the amount of friction between the moving components of the adjustment mechanism. By reducing friction, less force is required to move the seat assembly, thus less noise and vibration is produced. Furthermore, smaller and lighter weight components can be used. However, friction and inefficient gearing within the drive mechanism has a positive benefit of preventing back driving, i.e. unintended rotation of the lead screw caused by forces applied to the seat assembly. If the amount of friction in the drive mechanism is sufficiently reduced, it may be possible to back drive the adjustment mechanism by applying a load to the seat, causing the lead screw to rotate. The result is unintended movement of the seat relative to the vehicle body.
The present invention provides a brake within the transmission assembly to prevent rotation of the lead screw when a large force is applied to the seat assembly. The brake is only operated as a result of a high load applied to the seat assembly. During normal seat operation, the brake is not applied and the drive mechanism has the benefit of the low friction.
The transmission assembly of the present invention utilizes a coned-disk spring, such as a belleville washer, disposed between a radial shoulder formed by the lead screw and a radial shoulder formed by the transmission housing. The spring is frusto conical in shape in its normal state. When a load is applied to the seat causing movement of the transmission housing relative to the screw in the proper direction, the shoulder on the screw moves toward the housing shoulder. The coned-disk spring is flattened therebetween causing the spring to tightly grip the lead screw. Simultaneously, the outside diameter of the spring bears outwardly against the transmission housing. The result is a locking of the screw to the housing, thereby preventing rotation of the lead screw and back driving of the adjustment mechanism.
The transmission housing and lead screw are designed to slightly compress the coned-disk spring within its elastic range to put a pre-load on the spring and to bias the lead screw against a thrust bearing support in the transmission housing.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.